1. Field of the Invention
The present invention relates to well-drilling methods, and more particularly it relates to a method of construction of multiple-string wells in a troublesome rock drilling environment.
The present invention can be implemented to the utmost effect for shutting-off incompatible (as regards drilling conditions) beds, that is, beds with abnormal formation pressures, characterized by intense loss of circulation of the drilling mud and inflow of either a fluid or a gas from the exposed formation, as well as for shutting-off troublesome zones of caving-in or crumbling in a well, wherein the known methods are ineffective.
2. Description of the Related Art
There is known a method of construction, of multiple-string wells, wherein the rock-drilling operation is accompanied by flushing of the bottom hole to wash away the drillings. The formation pressures are counterbalanced by the pressure of the column of the washing fluid, controlled by varying its density. However, in many cases formations differ so broadly by the values of their pressures that the abovementioned balance would not be maintained by controlling the washing fluid density. In such cases, prior to drilling into the successive formation, the exposed formation is shut-off with the casing string which is subsequently cemented to avoid the inter-influence of the exposed formations, whereafter the washing fluid density is adjusted to correspond to the pressure of the successive formation, and the drilling is resumed, but with a bit of a smaller diameter. Consequently, the well in its extent from the head to the producing formation is of a stepped diameter diminishing in the downward direction, its wall being lined with several concentrically arranged casing strings. These strings are commonly called, in correspondence with their function, the conductor casing the surface casing string, the intermediate casing string and the flow string. It can be seen that this known technique involves a large input of time, cement and casings.
The desire to minimize the number of the intermediate casing strings by increasing the depth of drilling with a bit of one and the same diameter more often than not leads to additional troubles and even emergencies, such as caving-in or seizure of the drilling tool.
There is further known a method of construction of multiple-string wells (SU, A, No. 907220), including the steps of rock-drilling and consolidating the rock drilled with casing strings defining a conductor casing, a surface casing string intermediate casing strings and a flow string, according to which the zones of lost circulation of drilling mud, not detected at the project stage of the well construction, are shut off by being lined with profile pipes which are reamed while being set by building up a pressure drop across the pipes, followed by their expansion. This allows to shut off the zone of the loss of the washing fluid and continue the drilling operation, which is subsequently followed by setting the corresponding casing strings.
However, in this case, too, the structure of the well remains a multiple-string one, i.e. the last-described method of the prior art does not eliminate intermediate casing strings used for shutting off other troublesome zones in the drilling process. Therefore, this method also involves an increased input of time and materials into the construction of a well which is likewise of a diameter diminishing stepwise in the downward direction. Furthermore, the drilling of such a well requires frequent changes of drilling tools to correspond to the successively diminishing diameters, which further prolongs the drilling operation and makes it more costly, to say nothing of the industry being obliged to produce an unnecessarily large assortment of drilling tools and the associated equipment. It is an object of the present invention to provide a method of construction of multiple-string wells that would ensure casing-in of a well after the setting of the conductor casing and of the surface casing string with casing strings having the same inner diameter.